1. Field of the Invention
The present mention relates generally to the field of compression molding using molding materials that are composed of composite chips that contain unidirectional fibers and a tacky then resin matrix. More particularly, the invention provides an alternative to the type of compression molding process where the inherent tackiness of the composite chips is used to form a self-cohesive sheet of molding material that is cut and/or folded to form a self-cohesive free-standing body which is commonly known as a “preform”. Typically, multiple plies of the sheet molding material are used in making a single preform. The preform, which has a shape that matches the final shape of the desired part, is cured in a compression mold to form the final product.
2. Description of Related Art
Pre-impregnated composite material (prepreg) is used widely in the manufacture of composite parts and structures. Prepreg is a combination of uncured resin matrix and fiber reinforcement that is ready for shaping and curing into the final composite part. By pre-impregnating the fiber reinforcement with resin, the manufacturer can carefully control the amount and location of resin that is impregnated into the fiber network and ensure that the resin is distributed in the network as desired.
Unidirectional (UD) tape is a common form of prepreg. The fibers in unidirectional tape are continuous fibers that extend parallel to each other. The fibers are typically in the form of bundles of numerous individual fibers or filaments that are referred to as a “tows”. The unidirectional fibers are impregnated with a carefully controlled amount of uncured resin. The UD prepreg is typically placed between protective layers to form the final UD tape that is rolled up for storage or transport to the manufacturing facility. The width of UD tape typically ranges from less than one inch (2.5 cm) to a foot (30.5 cm) or more.
Unidirectional tape is not well-suited for use as a molding compound for forming complex three dimensional structures using compression molding techniques. The parallel orientation and continuous nature of the fibers in the UD tape cause fiber bunching or bridging when the UD tape is forced to fit the features of the complex part. As a result, the manufacture of complex three dimensional parts using UD tape has been limited to a laborious process where individual plies of UD tape are applied directly to a three dimensional mold, which is subsequently processed in an autoclave or other molding apparatus. This lay-up procedure using UD tape tends to be a long and costly process.
Molding compounds, which are generically referred to as discontinuous fiber composite (DFC) molding compound, have been found to be suitable for compression molding complex parts. One type of DFC molding compound is composed of random segments of individual chopped fibers that are combined with a thermosetting resin matrix. The randomly oriented chopped fibers more easily fit the features of a complex three dimensional part. However, the movement of the random fibers during high-pressure molding can vary unpredictably from one molded part to the next and may also differ between different features of a given part.
Another type of DFC molding compound, which is referred to herein as DFC sheet molding compound, combines the attributes of UD tape and randomly oriented short fibers into a single molding compound that can be accurately molded and machined to form a wide variety of relatively complex structures. DFC sheet molding compound is composed of randomly oriented segments or chips of unidirectional tape that have been impregnated with a tacky thermosetting resin. This type of quasi-isotropic DFC sheet molding compound was first used to make a variety of aerospace components. DFC sheet molding compound has also been a popular molding material for use in making the high strength molds that are used to compression mold the composite parts.
DFC sheet molding compound is available from Hexcel Corporation (Dublin, Calif.) under the trade name HexMC®. Examples of DFC sheet molding compound and the types of parts that have been made using sheet molding compound are described in U.S. Pat. Nos. 8,366,046; 7,510,390; 7,960,674 and published US Patent Application Nos. US2012-0040169-A1 and US2013-0101406, the contents of which are hereby incorporated by reference.
DFC sheet molding compound is typically made by laying multi-filamentary tows (yarns) parallel to each other on a suitable backing and impregnating the parallel tows with uncured thermosetting resin to form a UD prepreg. The UD prepreg is then chopped to form UD chips which are generally from 2 mm to 25 mm wide and from 25 mm to 125 mm long. The UD chips are then made into the DFC sheet molding compound, which is in the form of a layer of quasi-isotropically oriented UD chips. Typical thicknesses for a layer of DFC sheet molding compound range from 0.4 mm to 2 mm.
The inherent tackiness of the uncured thermosetting resin in the DFC sheet molding compound makes it possible to fold and manipulate one or more layers of the DFC sheet molding compound to form a complex three dimensional preform that is then compression molded to form the final composite part. The type of parts that are now being made using DFC sheet molding compound has expanded into a wide range of applications outside of the aerospace and mold making industries. Parts made by compression molding preforms composed of DFC sheet molding compound are now found in automobiles, motorcycles, bicycles and a wide range of other applications.